Speaker



March 1, 19 v I w. c. ROHN ETAL 2,926,741

SPEAKER Filed April 28, 1958 2 Sheets-Sheet l INVENTORS.

March 1, 1960 w, c. ROHN ET AL SPEAKER 2 Sheets-Sheet 2 Filed April 28,1958 United States Patent; 0. cc

SPEAKER William C. Rohn and Gordon P. McFaul, St. Joseph,

Micl1., assignors to Laboratory Equipment Corporation, St. Joseph,Mich., a corporation of Michigan Application April 28, 1958, Serial No.731,509

1 Claim. (Cl. 181-32) The present invention relates to speakers of thetype in which electrical energy representative of music, speech and thelike, is converted into acoustical energy.

One of the objects of the present invention is to provide a novelspeaker having a high fidelity of response together with high efficiencyand high power output.

An additional object is to provide an improved speaker having the abovecharacteristic and in which distortion introduced by the speaker is keptto an extremely low level.

Still another object is to provide an improved speaker particularlyadapted for operation in the low frequency portion of the soundspectrum.

Yet another object is to provide a novel speaker having an excellentresponse with low distortion throughout a wide frequency range and inwhich the high frequency portion of the sound spectrum is much lessdirectional in its characteristics than is usual.

Still another object is to provide a speaker having most or all of theabove characteristics and which in addition may be supplied in largesizes while at the same time maintaining an extremely shallow front toback dimensional requirement.

Still another object is to provide a speaker of the vibratory cone orpiston type, which is substantially without distortion, of the characterwhich is usually introduced by acoustical shadows. Other objects andadvantages will become apparent from the following description of apreferred embodiment of my invention which is illustrated in theaccompanying drawings.

In the drawings, in which similar characters of reference refer tosimilar parts throughout the several views:

Fig. 1 is a front viewof a speaker embodying features of the presentinvention;

Fig. 2 is a transverse medial sectional view through the speaker, whichmay be considered as taken in the direction of the arrow, along the line22 of Fig. 1;

Fig. 3 is a fractional view drawn to larger scale of a portion of thespeaker near the edge thereof with the view oriented as in Fig. l, butwith portions of the mechanism broken away so as to reveal theunderlying structure;

Fig. 4 is a large scale fractional transverse sectional view through therim portion of the speaker and may be considered as taken in thedirection of the arrows substantially along the line 4--4 of Fig. 3;

Fig. 5 is a view similar to Fig. 4 but taken in a slightly differentposition along the speaker rim as indicated by the line 55 of Fig. 3;

Fig. 6 is a transverse fractional sectional view showing a portion ofthe speaker at one stage of its manufacture;

Fig. 7 may be considered as a fractional side view of a portion of acoil form which may be used in constructing the speaker of the presentinvention; and

Fig. 8 is a transverse sectional view of a detail, and may be consideredas taken in the direction of the arrows along the line 8-8 of Fig. 7.

Speakers of the type to which the present invention relates are used forthe reproduction of sound. Briefly, an

2,926,741 Patented Mar. 1, 1960 undulating current is converted by thespeaker to mechanical vibratory motion and, by acoustical coupling, thisvibratory energy is transferred to the surrounding air mass. Speakers ofthis type usually assume the form of a relatively stiff cone or pistonformed of paper, pressed fiber, or a similar material which is supportedat its edge by means means of a flexible ring connected to acircumscribing framework. The flexible member, usually referred to asthe hinge, may take the form of a separate element provided specificallyfor this purpose, or, more commonly, this element is formed simply bycircumferentially corrugating the material of which the cone is formedso as to give it greater flexibility. The center of the cone is fixed toa voice coil which carries the undulating current, and this coil islocated within a relatively strong magnetic field so that the coil movesinwardly and outwardly within the field as the intensity and polarity ofthe coil current varies, and this motion in turn is transferred to thecone at its center. The mechanical motion of the cone thus produced istransferred to the surrounding air mass by the acoustic coupling betweenthe cone face and the air.

In the design of speakers of this general character there are severalcontrolling limitations, many of which are mutuallyincompatible.Ordinary engineering in this field, therefore, consists in compromisingthe several design factors so as to achieve a reasonably effectivebalance. A speaker having the features of our invention, however,achieves a much higher than usual excellence, since some at least of theincompatible limiting factors may be eliminated from consideration, aswill appear. As an example, it is known, among other things, that themagnetic field assembly should be relatively massive and rigidlysupported so as to provide considerable flux and low vibration. Thisdesign characteristic in ordinary speakers is incompatible with theelimination of as much structure behind the cone as possible in order toreduce acoustic shadows. The present invention eliminates this problemcompletely.

The speaker of the present invention although it has good frequencyresponse throughout the audible spectrum is particularly intended foruse as an extremely high quality, low frequency transducer in a soundsystem. Such speakers are commonly referred to as woofers." Theembodiment shown is intended for efficient and smooth operation at highpower levels and, as illustrated, is approximately 18 inches in overalldiameter. In general, it consists of a vibratory cone or piston 10connected at its periphery to a hinge ring 12 which projects radiallyoutwardly. The outer edge portion of this hinge is clamped between afront ring at 14 and a back ring at 16, these two rings in turn beingsecured to an annular mild steel plate 18 near its outer edge. The plate18 extends inwardly so that its inner edge 20 is located slightlyoutwardly of a position of alignment with the periphery 22 of the cone10. The plate 18 and the rings 14 and 16 are secured together by severalrather closely spaced, circumferentially arranged screws 24 which passthrough the rings and are threaded into the plate 18.

The structure consisting of the rigidly bound together plate 18 andrings 14 and 16 constitute the mounting flange for the speaker so thatthe speaker can be secured to its bafiie board by screws passed throughholes 26 which in the embodiment shown are arranged so that the holes 26alternate with the screws 24 around the periphery of the structure.

The voice coil, represented by the numeral 28, consists of a coil ofwire indicated at 30 wound upon the rearward portion of a large diameterrelatively short coil form 32. This coil form has a diameter equal tothat of the piston 10 and is secured to the peripheral edge of thepiston between the piston and hinge 12. The voice coil extendsrearwardly of the piston so as to place the coil 30 wound thereon withina gap 34 in a magnetic circuit.

The magnetic structure forming this gap consists of the and thencestraight outwardly so as to provide a forward fiat face 38 which isparallel to the rearward face 39 of the ring 18. These parallel flatfaces in the present embodiment are spaced apart a distance of about oneand one-half inches.

The two elements 36 and 18 are connected together by means of relativelylong screws 40 which pass through the plate 18 and are threaded into thering 36. Between each of these screw locations a magnetic slug 41 ispositioned so that the slugs extend all the way around the structure inside-by-side relationship. The slugs are formed of cast Alnico 5 orequivalent material, and since they are clamped in place by endwisepressure applied between parallel faces, they need only to be ground touniform length with parallel end surfaces.

More particularly, the cone or piston 10 is molded of Styrofoam. Thispiston has considerable thickness and has relatively smooth faces with acellular interior. It is therefore extremely light, while at the sametime being extremely stiff. This stiffness is enhanced by giving thepiston a compound parabolic contour with the central portion of thepiston being folded forwardly so as to present a convex surface as isbest seen in Fig. 2. Although the following dimensions need not beclosely adhered to, the piston shown, which is representative, is about15 inches in diameter. The diameter of the forwardly projecting portionat the center is about one-half the total piston diameter, and the totaloverall front to back dimension of the speaker is about 3% inches. Thematerial in the cone is graduated in thickness so as to be thickest atthe center and to thin out gradually as it approaches the foldedportion. Outwardly of the fold, it gradually increases in thickness andthen thins down somewhat just before reaching the short cylindricalportion at the periphery which is cemented to the voice coil form. Asthus contoured, great strength and stiffness is obtained with minimumweight.

In addition to contributing considerably to the overall stiffness,folding the piston also contributes greatly to the angular dispersal ofthe high frequency sound radiated therefrom. This effect is broughtabout by the rather sharply curved forward surface of the dome at thecenter of the piston and is aided further by the conical, rather thanparabolic, forward face on the portion outwardly of the fold. Althoughthe forward face on the portion outwardly of the fold is conical, theback surface thereof is curved such that this section of the piston as awhole, in other words, along the centerline of the section, issubstantially parabolic.

The periphery of the cone is molded so as to be substantiallycylindrical and is secured to the voice coil form 28 by cementing. Thiscoil form may be comprised of a strip of parchment paper. After the formis cemented to the piston, it is slid over the end of a coil windingarbor 42, see Fig. 6. Preferably in order to promote the subsequentseparation of the coil form from the arbor 42, a thin layer, such as aone mil thickness, of polyethylene is interposed first between the arborand the coil form. No.

27 copper wire, polyurethane coated, is suitable for the coil 36. Othersizes can, of course, be substituted, and the insulating material usedis largely a matter of choice. This coil should be well cemented to thecoil form, since under conditions of high power operation, the coil wirewill be torn loose from the coil form if its adhesion thereto is poor. iprefer to accomplish the adhesion by coating the exterior surface of thecoil form with a suitable adhesive, such as an epoxy resin, and then towind the wire directly into the layer of adhesive. ii

The hinge 12 is preferably a ring die cut from a sheet of polyurethanefoam which is approximately a of an inch thick. This material iscellular in nature and although it is rather flexible, it has a veryconsiderable resistance to stretching and resists taking a permanentset. The outer portion of the polyurethane foam hinge is clamped betweenthe rings 14 and 16 as previously described, and therefore the materialof the hinge at its periphery is squeezed flat with an accompanyingcollapse of the interior cellular structure. The result is that thehinge 12 has great resistance to transverse movement in any directionand the piston therefore is well maintained in a centered locationrelative to the framework and the magnetic gap, but the hinge has littleresistance to bending, particularly at the pinched portion of the hingewhich is just inwardly of the clamping rings. The piston therefore hasgreat freedom of motion in an inwardly and outwardly direction butcannot move in any other direction.

Since the hinge maintains the cone 10 in precise alignment, the gap inthe magnetic circuit within which the voice coil operates, may be quitesmall. This is important since it helps in establishing a high magneticflux density at the gap. Although the large diameter parchment voicecoil 28 could ordinarily expect to be subject to considerabledeformation in use, we have found that once it is placed upon the arbor42 and the wire coil 30 has been wound thereon and tightly cemented inplace, the wire acts as a reenforcing element for the voice coil form sothat thereafter the voice coil maintains its shape and dimensions quiteprecisely in spite of small changes which may take place in the diameterof the piston over a period of time.

If it is desired to use still higher flux densities, or less magneticmaterial in the circuit, or if for any reason an even smaller gap isdesired and hence even more precise alignment and dimensional stabilityof the voice coil is called for, we have found that an excellentarrangement is to make the voice coil form from a strip of thinmagnesium ribbon, as is illustrated in Fig. 7, in place of theparchment. This magnesium ribbon is cemented to the rim of the piston inthe same manner as the parchment strip, excepting that the ends of theribbon 59 should not touch each other. This is illustrated by the slightgap at 52. This gap prevents the coil form from acting as a single turn,short circuited coil within which a current would be induced duringoperation of the speaker. As shown in Fig. 7, the ends of the ribbon areskived, and although it would appear that the portion of the ribbonprojecting beyond the edge of the piston would be comparatively weak, ithas been found that once the wire 30 has been wound and cementedthereon, the wire acts as a sufiicient bridge across the gap 52 to givethe required rigidity.

If the speaker is to be used in very high power applications where thetendency of the alternating current in the coil 39 to strip the wirefrom the form is great enough to cause some failures in service, thiscan be rectified by rolling the extreme free edge of the magnesiumribbon 50 outwardly slightly to form a slightly hooked edge as at 54 inFig. 8. By winding one of the turns against this hooked edge and theother turns in contact therewith and with each other, the resistance tostripping of the Wire from the coil form is greatly increased.

It should be noted that the clamping ring 16 is cut away at the back asat 56 so as to provide large openings communicating between theatmosphere and the space behind the hinge 12. This is advisable since itprevents pressure variations in this space from having a damping actionupon the piston.

Ordinarily in speaker construction the voice coil and the materialforming the magnetic circuit are located directly behind the center ofthe cone and are supported from the rim by several arms or similarstructure. This structure directly. behind and close to the coneproduces acoustic shadows and reflections and introduces distortionwhich under some conditions may be quite serious. The difliculty iscompounded by the fact that the arrangement for forming the magneticcircuit and magnetic gap must be very rigidly supported from the rim sothat the forces tending to pull the voice coil into the magnetic gapwill produce movement of the voice coil and cone only without causingsubstantial vibration in the elements of the magnetic circuit.

An important feature of this invention is that, as will be noted, thematerial forming the permanent magnet circuit is at the extremeperiphery of the piston and no portion thereof projects inwardly beyondthe edge of the piston more than a very small amount, of the order of /1to of an inch at most. Thus, the central hole through the speaker frameand magnetic circuit defined by the cylindrical portion 36 of theL-shaped ring is almost as large as the speaker piston. Thissubstantially completely eliminates acoustical'shadows and reflections.The distortion is therefore less and both sides of the speaker can beused for high quality sound radiation. Furthermore, the mechanismforming the magnetic circuit is extremely rigidly supported relative tothe mounting flange.

Because of the folded nature of the piston, and because the magneticmaterial and the voice coil are not behind the center of the cone, thedepth of the speaker from front to back is extremely shallow. This is animportant advantage in mounting the speaker under certain commonlyencountered conditions.

From the above description of the invention it will be appreciated thatmany worthwhile objectives have been accomplished which in the main areas follows. The piston is extremely light, yet extremely stifl. It iswell centered and supported, but nevertheless because of the high degreeof flexibility of the pinched portion of the hinge, the naturalfrequency of oscillation of the piston is low and therefore the speakerwill respond properly down to the low end of the audibile spectrum sincethroughout the frequency range of operation, the resonant frequency ofoscillation of the piston is not reached.

Because the cone is well centered and because of the high degree ofrigidity of the frame, a very small flux gap can be used, and it ispossible to obtain extremely high flux densities, particularly sincemagnetic slugs may be used all the way around the periphery of thespeaker, thereby enabling a large quantity of magnetic material to beinserted into the magnetic circuit without obstructing the back of thepiston. As previously discussed, the forward face of the pistondisperses the sound, including the usually high directional highfrequencies over a wide angular range, and even under conditions wherethe speaker is wall mounted between two rooms the output of the speakeron the two sides is substantially the same, particularly in the lowerfrequency range, since there is no structure which can produce acousticshadows on the back side of the speaker.

Furthermore, the speaker is so extremely thin from front to back, inspite of the fact that the example shown is an 18 inch speaker, thatmounting in an ordinary wall with grills or other coverings on each sideis entirely feasible.

It will be understood that the above specific description of arepresentative embodiment of our invention should not be considered aslimitative, since variations may be made in the structure withoutdeparting from the scope or spirit of the invention, and that the scopeof the invention is to be determined from the scope of the followingclaim.

Having describer our invention, what we claim as new and useful anddesire to protect by Letters Patent of the United States is: t

A speaker piston formed of lightweight stiff cellular material having anappreciable thickness, said piston having a circular periphery, thematerial of said piston being formed to a contour such that a transversemedial section therethrough constitutes a compound parabola with theforward face of the outer portion of said piston being concave and thecentral portion thereof being reversely curved so as to be convex, saidconcave and convex portions being joined by a smoothly blended curve ofsmall radius, and the thickness of said cellular material being greatestat the center and tapering to a thinner section where the centralportion joins said small radius curve, and the thickness of said outerportion tapering from a thin section adjacent said small radius curve toa thicker intermediate section and thence to a thinner section adjacentsaid periphery.

References Cited in the file of this patent UNITED STATES PATENTS1,844,657 Hinckley et a1 Feb. 9, 1932 1,868,090 Bostwick July 19, 19322,538,621 Hopkins et al Jan. 16, 1951

